Sorry, I meant to reply to this with basically the link Peter offered (thanks for the link Peter) and my analysis of the link yesterday evening. No excuses, I simply forgot to get back to it.
As for the analysis, I guess I can still offer that. As the chart shows, the men’s marathon record has dropped in steps. Looking at the past 55-60 years, you had a significant drop through the mid 1950s after staying relatively steady for nearly 2 decades. Then, it leveled off some until the early to mid 1960s, when it dropped again right through the late 1960s. It then leveled off quite a bit again until a bit of the drop in the mid 1980s. It then held steady again, without even a single record for a full decade, until some recent movement over the past 6 years.
A lot of people look at the general trend over the past century and try to say the 2:00 barrier will be broken in the next decade. However, look at the trends and you will see the stepped progression that, if a statistician were trying to fit a curve to it, would best fit an inverted logarithmic curve. That means things are greatly leveling off, as we can see by looking at more recent numbers. Interestingly, the progression over the past 15-20 years has accelerated over roughly the same time period previous to that but this is a progression that should not be expected to continue (by clean athletes at least).
From 1969 to 2003, a 34 year timespan, 3:38 was taken off the world record. If the progression continued at that rate, we could expect the 2:00 barrier to be reached in 2049.
From 1985 to 2003, an 18 year timespan, 3:17 was taken off the world record. If the progression continued at that rate, we could expect the 2:00 barrier to be reached in 2030.
From 1988 to 2003, a 15 year timespan, 1:55 was taken off the world record. if the progression continued at that rate, we could expect the 2:00 barrier to be reached in 2041.
If we wanted to believe trends would continue, taking the average would suggest that the 2:00 barrier would be reached roughly in 2040. However, the greater trend in all world record progressions, from the 100 to the marathon, has been that the rate of progression has in general slowed. This is why I say that I would be more surprised to see the 2:00 barrier broken before I’m an old man than I would be to not see it broken at all in my lifetime.